{"gene":"IMPDH2","run_date":"2026-06-10T01:55:23","timeline":{"discoveries":[{"year":2020,"finding":"Cryo-EM structures of human IMPDH2 in active and inactive conformations define the mechanism of filament assembly: polymerization stabilizes conformations that make the enzyme less sensitive to feedback inhibition by GTP, explaining why assembly occurs when guanine nucleotide pool expansion is required. Filament-dependent allosteric regulation (tuning inhibitor sensitivity) distinguishes IMPDH2 from other metabolic filaments.","method":"Cryo-EM structural determination of IMPDH2 in multiple conformational states, correlated with enzymatic activity measurements","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures at multiple conformational states with functional validation of allosteric regulation mechanism in a single rigorous study","pmids":["31999252"],"is_preprint":false},{"year":2017,"finding":"IMPDH2 is an intracellular target of the cyclophilin A (PPIA)–sanglifehrin A (SFA) binary complex: the ternary PPIA-SFA-IMPDH2 complex forms through the CBS (cystathionine-β-synthase) domain of IMPDH2, does not inhibit IMPDH2 enzymatic activity, but modulates cell growth. The SFA complex is highly isoform-selective for IMPDH2 over IMPDH1.","method":"Affinity pulldown, co-immunoprecipitation, domain mapping (CBS domain), cell growth assays, isoform selectivity experiments","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal interaction identified with pulldown and Co-IP, domain mapped to CBS domain, enzymatic activity tested, isoform selectivity demonstrated in one study with multiple orthogonal methods","pmids":["28076787"],"is_preprint":false},{"year":2019,"finding":"ANKRD9 facilitates degradation of IMPDH2 (the rate-limiting GTP biosynthesis enzyme). Under basal conditions ANKRD9 is segregated from cytosolic IMPDH2 in vesicle-like structures; upon nutrient limitation, ANKRD9 loses its vesicular pattern and assembles with IMPDH2 into rod-like filaments where IMPDH2 is stable. Inhibition of IMPDH2 activity with ribavirin favors ANKRD9 binding to IMPDH2 rods; guanosine supplementation reverses this. The conserved Cys109-Cys110 motif in ANKRD9 is required for the vesicle-to-rods transition and for binding/regulation of IMPDH2. ANKRD9 knockdown increases IMPDH2 levels and prevents IMPDH2 rod formation upon nutrient limitation.","method":"Co-immunoprecipitation, fluorescence microscopy, subcellular fractionation, site-directed mutagenesis (Cys109/110), ANKRD9 knockdown and overexpression, guanosine rescue experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, mutagenesis of required motif, multiple rescue conditions, live-cell imaging, single lab with multiple orthogonal methods","pmids":["31337707"],"is_preprint":false},{"year":2023,"finding":"Disease-associated point mutations in IMPDH2 linked to dystonia and neurodevelopmental disorders disrupt GTP-mediated allosteric inhibition of the enzyme. Cryo-EM structures of one IMPDH2 mutant reveal this regulatory defect arises from a shift in the conformational equilibrium toward a more active state, identifying gain-of-function dysregulation as the disease mechanism.","method":"Cryo-EM structural determination of mutant IMPDH2, enzymatic activity assays measuring GTP inhibition, patient variant identification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure of disease mutant with enzymatic functional validation; single lab but two orthogonal methods (structure + enzyme assay)","pmids":["37414152"],"is_preprint":false},{"year":2024,"finding":"IMPDH2 serine 122 (S122) is phosphorylated by CDK1; this modification attenuates IMPDH2 catalytic activity for IMP oxidation and represses its allosteric modulation by purine nucleotides. FGFR signaling activation triggers IMPDH2-S122 dephosphorylation mediated by protein phosphatase 2A (PP2A); this is dependent on FGFR3-mediated phosphorylation of PPP2R1A at Y261, leading to formation of a PPP2CA–PPP2R1A–IMPDH2 complex. Dephosphorylation at S122 activates IMPDH2, promotes guanine nucleotide synthesis, facilitates S-phase completion and cell proliferation.","method":"In vitro kinase assay (CDK1), phosphoproteomics, co-immunoprecipitation (PP2A complex), site-directed mutagenesis (S122), enzymatic activity assays, FGFR signaling manipulation","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro kinase assay, mutagenesis, co-IP of phosphatase complex, enzymatic activity assays; multiple orthogonal methods in single study","pmids":["39739531"],"is_preprint":false},{"year":2024,"finding":"IMPDH2 is enriched on chromatin in triple-negative breast cancer (TNBC). On chromatin, IMPDH2 interacts with PARP1 and modulates PARP1 activity by controlling the nuclear availability of NAD+, fine-tuning the DNA damage response. IMPDH2 repression leads to DNA damage accumulation; when IMPDH2 is restricted to the nucleus, it depletes nuclear NAD+, leading to PARP1 cleavage and cell death.","method":"Chromatin fractionation, co-immunoprecipitation (IMPDH2-PARP1), NAD+ measurement in nuclear fractions, IMPDH2 knockdown/nuclear restriction experiments, DNA damage assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP, subcellular fractionation, NAD+ quantification, KD with specific phenotypic readout; multiple orthogonal methods in single study","pmids":["39532854"],"is_preprint":false},{"year":2025,"finding":"USP5 (ubiquitin-specific protease 5) binds IMPDH2 through its N-terminal cryptic ZnF-UBP and ZnF-UBP domains interacting with IMPDH2 residues 251–514, and removes K48-linked ubiquitin chains from IMPDH2, preventing ubiquitin-mediated proteasomal degradation and stabilizing IMPDH2 protein levels.","method":"Co-immunoprecipitation, domain mapping, ubiquitination assay, USP5 knockdown/overexpression, proteasome inhibitor experiments","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP with domain mapping, ubiquitination assay showing K48 chain removal, single lab","pmids":["40164869"],"is_preprint":false},{"year":2024,"finding":"JOSD2 (a deubiquitinating enzyme) binds the C-terminal domain of IMPDH2 and preferentially cleaves K63-linked polyubiquitin chains at the K134 site of IMPDH2, suppressing IMPDH2 activity and preventing activation of NF-κB signaling and inflammation in macrophages. JOSD2 deficiency exacerbates colitis through enhanced IMPDH2-mediated NF-κB activation.","method":"Co-immunoprecipitation, in vitro deubiquitination assay (K63 chain specificity, K134 site identification), myeloid-specific JOSD2 knockout mice, NF-κB reporter assays","journal":"Acta pharmaceutica Sinica. B","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, in vitro deubiquitination assay with ubiquitin chain type and site specificity identified, conditional KO mouse model; single lab","pmids":["40177575"],"is_preprint":false},{"year":2022,"finding":"SARS-CoV-2 Nsp14 protein physically interacts with IMPDH2, and IMPDH2 knockdown or chemical inhibition abolishes Nsp14-mediated NF-κB activation and downstream cytokine (IL-6, IL-8) induction, demonstrating that IMPDH2 is required for Nsp14-driven NF-κB signaling and is exploited for viral replication.","method":"Co-immunoprecipitation (Nsp14–IMPDH2 interaction), siRNA knockdown of IMPDH2, NF-κB reporter assays, IMPDH2 inhibitors (ribavirin, mycophenolic acid), viral replication assays","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP confirmed interaction, genetic and pharmacological inhibition with specific signaling readout; single lab, multiple methods","pmids":["36177032"],"is_preprint":false},{"year":2015,"finding":"Microinjection of affinity-purified anti-IMPDH2 antibodies into live COS-7 cells caused disassembly of rod/ring (RR) structures, demonstrating that IMPDH2 is a major structural component and essential building block of cytoplasmic RR structures. RR structures are primarily stationary and stable in live cells. GFP-tagged IMPDH2 confirmed these findings independently.","method":"Microinjection of anti-IMPDH2 antibodies in live cells, live-cell fluorescence microscopy, GFP-IMPDH2 transfection, time-lapse imaging","journal":"Cell & bioscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional manipulation (antibody microinjection) in live cells with real-time imaging, two independent methods (antibody injection and GFP fusion); single lab","pmids":["25601894"],"is_preprint":false},{"year":2015,"finding":"Conditional deletion of Impdh2 in early neural crest cells (using Wnt1-Cre) produces highly penetrant intestinal aganglionosis, agenesis of craniofacial skeleton, and cardiac outflow tract/great vessel malformations, establishing that IMPDH2-mediated guanine nucleotide synthesis is essential for enteric nervous system development and other neural crest derivatives.","method":"Conditional knockout mouse (Wnt1-Cre × Impdh2-flox), Rosa26 reporter lineage tracing, histological and developmental analysis","journal":"Developmental biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — tissue-specific conditional KO with defined neural crest phenotypes, lineage tracing; well-controlled in vivo genetics","pmids":["26546974"],"is_preprint":false},{"year":2020,"finding":"IMPDH2 expression is necessary for activation of de novo GTP biosynthesis in glioblastoma; increased IMPDH2 enhances RNA polymerase I and III transcription, directly linking GTP metabolism to anabolic capacity and nucleolar enlargement.","method":"IMPDH2 knockdown/overexpression, GTP level measurement, RNA polymerase I/III activity assays, nucleolar size analysis","journal":"Journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — KD with specific metabolic and transcriptional phenotype, single lab review-style paper summarizing own findings; limited methodological detail in abstract","pmids":["32702086"],"is_preprint":false},{"year":2023,"finding":"FANCI directly binds IMPDH2 (by co-immunoprecipitation) and decreases IMPDH2 degradation; overexpression of IMPDH2 reverses the inhibitory effects of FANCI knockdown on lung adenocarcinoma cell proliferation via MEK/ERK/MMPs signaling activation.","method":"Co-immunoprecipitation, immunofluorescence co-localization, FANCI knockdown, IMPDH2 overexpression rescue, western blotting of MEK/ERK/MMP pathway","journal":"OncoTargets and therapy","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single Co-IP with functional rescue experiment; pathway placement by western blot; single lab","pmids":["32021289"],"is_preprint":false},{"year":2015,"finding":"IMPDH2 and CTPS co-assemble into cytoplasmic rod/ring (RR) structures in a cell-type- and induction-dependent manner. Ribavirin and mycophenolic acid (MPA) treatments induce exclusively IMPDH2-based RR, while DON treatment produces mixed IMPDH2/CTPS RR. Assembly composition is determined by which biosynthetic pathway is inhibited.","method":"Immunofluorescence microscopy, drug treatments (ribavirin, MPA, DON), HA-tagged CTPS1 transfection, quantification of RR composition across cell lines","journal":"Journal of genetics and genomics","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct imaging with multiple pharmacological conditions and transfection approaches; replicated across two cell types (HeLa and COS-7)","pmids":["26165495"],"is_preprint":false},{"year":2024,"finding":"In mouse models of AMPD2 deficiency, neurodegeneration-resistant brain regions accumulate micron-sized IMPDH2 filaments while the vulnerable hippocampal dentate gyrus shows barely detectable filaments. Blocking IMPDH2 polymerization using a dominant-negative IMPDH2 variant impairs growth of AMPD2-deficient neural progenitor cells, suggesting IMPDH2 filament assembly protects against GTP deprivation-induced neurodegeneration.","method":"Mouse knockout model (AMPD2 deficiency), immunofluorescence for IMPDH2 filaments, dominant-negative IMPDH2 variant expression in human neural progenitor cells, GTP level measurement","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo mouse model with regional filament quantification and human cell dominant-negative experiment; single lab","pmids":["39075237"],"is_preprint":false},{"year":2023,"finding":"EBV EBNA2 and MYC (but not LMP1) induce IMPDH2 gene expression during primary B cell infection, leading to nucleolar hypertrophy. IMPDH2 induction and nucleolar enlargement are required for EBV-driven growth transformation of primary B cells, as inhibition by mycophenolic acid (MPA) blocks transformation.","method":"RNA-seq (IMPDH2 induction kinetics), EBNA2/LMP1 knockout EBV viruses, MPA pharmacological inhibition, electron microscopy of nucleolar size, mouse xenograft model with oral MMF","journal":"Microbiology spectrum","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic (viral KO) and pharmacological approaches to define upstream regulator and functional requirement; multiple methods in single lab","pmids":["37409959"],"is_preprint":false},{"year":2025,"finding":"IMPDH2 interacts with SRPK1 in CD4+ T cells (confirmed by mass spectrometry, co-immunoprecipitation, and confocal colocalization); IMPDH2 regulates SRPK1 expression, which subsequently activates the AKT/mTOR signaling pathway to promote CD4+ T cell proliferation and suppress apoptosis in myasthenia gravis.","method":"Mass spectrometry, co-immunoprecipitation, confocal fluorescence microscopy, IMPDH2 knockdown/overexpression, western blotting of AKT/mTOR pathway, flow cytometry","journal":"Clinical immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — MS + Co-IP + colocalization identify SRPK1 as interactor; pathway placement by western blot; single lab","pmids":["41213488"],"is_preprint":false},{"year":2020,"finding":"Shikonin is a competitive inhibitor of IMPDH2 (Lineweaver-Burk plot), directly interacts with IMPDH2 protein (thermal shift assay and fluorescence quenching), and its anti-proliferative effects in TNBC cells are rescued by exogenous guanosine supplementation, confirming on-target mechanism through GTP biosynthesis inhibition.","method":"Enzymatic inhibition assays (Lineweaver-Burk), thermal shift assay, fluorescence quenching, molecular docking, guanosine rescue in cell proliferation assays, IMPDH2 knockdown","journal":"Phytotherapy research","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro enzymatic assay with mechanism determination, biophysical binding assays, guanosine rescue; multiple orthogonal methods; single lab","pmids":["32779300"],"is_preprint":false},{"year":2010,"finding":"The IMPDH2 allozyme Phe263 has decreased enzymatic activity primarily due to decreased protein quantity caused by accelerated protein degradation, as supported by X-ray crystal structure analysis of IMPDH2. A promoter SNP (G>C IVS1-93) is associated with decreased mRNA quantity, possibly through altered transcription.","method":"Resequencing, allozyme functional assays, protein degradation experiments, X-ray crystal structure of IMPDH2, real-time PCR for mRNA quantification","journal":"British journal of pharmacology","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — crystal structure available plus protein stability assays; functional consequence of variant defined mechanistically; single lab","pmids":["20718729"],"is_preprint":false},{"year":2009,"finding":"A promoter SNP in IMPDH2 (c.-95T>G) disrupts a cAMP response element (CRE) binding site, leading to a 55–65% decrease in IMPDH2 transcriptional activity as measured by luciferase reporter assays in HEK293 and Jurkat cells.","method":"Luciferase reporter gene assays with IMPDH2 promoter constructs (wild-type vs. mutant) transfected into HEK293 and Jurkat cell lines","journal":"Genetic testing and molecular biomarkers","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct promoter-reporter functional assay in two cell lines; single lab, single method type","pmids":["19810816"],"is_preprint":false},{"year":2024,"finding":"IMPDH2 cytoophidia (filaments) form in mouse oocytes in vivo during growth and in vitro upon IMPDH inhibition with MPA. In granulosa cells, cytoophidia form after the LH surge coinciding with reduced IMPDH2 protein expression and resumption of oocyte meiosis. MPA-induced cytoophidium formation in cumulus cells triggers meiosis resumption in enclosed oocytes via the MTOR pathway; guanosine supplementation prevents this, linking IMPDH2 filament formation to regulation of oocyte meiotic competence.","method":"Immunofluorescence in mouse ovarian sections, MPA treatment of cumulus-oocyte complexes (COCs), LH surge induction, guanosine rescue, MTOR pathway inhibition, meiotic resumption assay","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo and in vitro imaging with pharmacological manipulation and pathway rescue; single lab with multiple conditions","pmids":["34124077"],"is_preprint":false},{"year":2025,"finding":"ALK and SRC kinases phosphorylate IMPDH2 at tyrosine 233 (Y233) within the Bateman domain, and this phosphorylation is essential for IMPDH2 enzymatic activity. Phosphoinositide-3-phosphate (PI3P) selectively binds to and inhibits IMPDH2 but not IMPDH1, revealing an isoform-specific lipid-mediated regulatory mechanism.","method":"Peptide-based mutagenesis, in vitro enzymatic assays, PI3P binding assay, kinase activity assays (ALK, SRC), isoform comparison (IMPDH1 vs IMPDH2)","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 1 / Weak — preprint with in vitro enzymatic and binding assays; not yet peer-reviewed; single study","pmids":[],"is_preprint":true},{"year":2024,"finding":"Impdh2 deletion in mouse myeloid lineage cells (LysM-Cre conditional KO) reduces osteoclast number and impairs osteoclast differentiation marker gene expression (Nfatc1, Ctsk, Calcr, Acp5, Dcstamp, Atp6v0d2), with genome-wide analysis showing impairment of mitochondrial biogenesis and oxidative phosphorylation in Impdh2-deficient osteoclasts. Deletion alleviates ovariectomy-induced bone loss.","method":"Myeloid-specific conditional KO (Impdh2-LysM-Cre), histomorphometric analysis, qPCR/western blot, genome-wide transcriptomic analysis, ovariectomy mouse model","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-type-specific conditional KO with defined osteoclast phenotype, genome-wide pathway analysis, in vivo bone loss model; single lab","pmids":["38959733"],"is_preprint":false},{"year":2024,"finding":"Vitexicarpin directly binds IMPDH2 (confirmed by CETSA, Biacore, MST) and promotes c-Myc ubiquitination by disrupting the protein-protein interaction between IMPDH2 and c-Myc, thereby inhibiting EMT in colorectal cancer. Co-IP confirmed a direct IMPDH2–c-Myc interaction.","method":"CETSA, label-free quantitative proteomics, Biacore, Co-IP, MST, ubiquitination assay, molecular docking, IMPDH2 overexpression/knockdown","journal":"Phytomedicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biophysical binding methods plus Co-IP for IMPDH2-c-Myc interaction; functional consequence via ubiquitination assay; single lab","pmids":["39008915"],"is_preprint":false},{"year":2026,"finding":"HSPA6 interacts with IMPDH2 and activates it by recruiting ROCK2 to phosphorylate IMPDH2 at serine 416 (S416), thereby promoting GTP synthesis, reducing DNA damage, and enhancing radioresistance of glioblastoma stem cells.","method":"Co-immunoprecipitation (HSPA6–IMPDH2, ROCK2 recruitment), in vitro kinase assay (ROCK2 phosphorylation of IMPDH2 S416), enzymatic activity assays, IMPDH2 knockdown/inhibition with irradiation, mouse GBM model","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — kinase assay with identified phosphorylation site, Co-IP of complex, enzymatic activity, in vivo validation; single lab with multiple methods","pmids":["42234559"],"is_preprint":false},{"year":2025,"finding":"IMPDH2 S160del mutant (neurodevelopmental disorder-associated) disrupts in vivo IMPDH2 filament assembly in Xenopus tropicalis by destabilizing dimerization of regulatory Bateman domains, as shown by cryo-EM structures. S160del expression disrupts purine pools and somite organization. Restoring Bateman domain dimerization with a high-affinity ligand rescues filament formation but does not restore GTP inhibition sensitivity, indicating S160del disrupts both filament assembly and allosteric regulation independently.","method":"Cryo-EM structure of S160del mutant, Xenopus tropicalis expression (WT vs. S160del), metabolomics (purine pools), developmental phenotyping (somitogenesis), filament visualization, high-affinity ligand rescue","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure, in vivo vertebrate model, metabolomics, ligand rescue; multiple orthogonal methods; single lab with rigorous design","pmids":["41343675"],"is_preprint":false},{"year":2024,"finding":"IMPDH2 forms localized spots (not diffuse filaments) predominantly at branching sites and distal ends of astrocyte stem processes in adult mouse hippocampus and primary astrocyte cultures, suggesting a role in site-specific GTP availability regulation at specialized astrocyte nodes.","method":"Immunofluorescence microscopy of adult mouse hippocampal sections and primary astrocyte cultures, quantitative distribution analysis","journal":"Genes to cells","confidence":"Low","confidence_rationale":"Tier 3 / Weak — localization by immunofluorescence only; functional consequence not directly tested; single lab, single method","pmids":["38009721"],"is_preprint":false},{"year":2024,"finding":"In Xenopus tropicalis tadpoles, inhibition of IMPDH2 impairs tail regeneration and reduces cell proliferation in regenerating tissue. Endogenous and GFP-tagged IMPDH2 robustly assembles into filaments throughout the tadpole tail, and the regenerating tail creates a sensitized condition for filament formation compared to non-regenerating tissue.","method":"IMPDH2 inhibitor treatment during tail regeneration, proliferation assay (EdU), endogenous filament immunofluorescence, GFP-IMPDH2 fusion live imaging in tadpole tail","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 2 / Weak — preprint; direct inhibition with proliferation readout and filament imaging in vivo; not yet peer-reviewed","pmids":["39131357"],"is_preprint":true},{"year":2024,"finding":"Impdh2 conditional knockout in adipocyte precursor cells (APCs) reduces white adipose tissue expansion under high-fat diet by limiting mitotic clonal expansion (MCE) during adipogenesis. XMP (xanthosine monophosphate, an IMPDH2 product) enhances adipogenic potential; exogenous XMP rescues the reduced adipogenic ability of Impdh2-deficient APCs, mechanistically linking Impdh2 activity to APC proliferation through nucleotide provision.","method":"APC-specific conditional Impdh2 knockout, high-fat diet model, XMP supplementation rescue, adipogenesis assays, energy expenditure measurement","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO with metabolite rescue experiment defining mechanism; single lab, in vivo and in vitro","pmids":["38692012"],"is_preprint":false},{"year":2023,"finding":"B7-H3 (CD276) co-immunoprecipitates with IMPDH2 in lung cancer cells; this complex protects cancer cells from oxidative stress triggered by chemotherapy, promoting cell survival. Loss of B7-H3 promotes 3D spheroid expansion in an IMPDH2-dependent manner, establishing a functional interaction between B7-H3 and IMPDH2 in metabolic homeostasis.","method":"Co-immunoprecipitation (B7-H3–IMPDH2), oxidative stress assays, B7-H3 knockdown with 3D spheroid growth, IMPDH2-dependent rescue experiments","journal":"Cancers","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP with limited mechanistic follow-up; single lab, single interaction method","pmids":["37444640"],"is_preprint":false}],"current_model":"IMPDH2 is the rate-limiting enzyme catalyzing the first committed step in de novo guanine nucleotide (GTP) biosynthesis; it is allosterically inhibited by GTP and regulated by reversible polymerization into filaments that reduce sensitivity to GTP feedback inhibition. Its activity is controlled by multiple post-translational mechanisms: CDK1 phosphorylates S122 to attenuate activity (reversed by FGFR-PP2A signaling), ROCK2 (recruited by HSPA6) phosphorylates S416 to activate it, ALK/SRC phosphorylate Y233 for activity, and K48-linked ubiquitination targets it for proteasomal degradation (counteracted by deubiquitinases USP5 and JOSD2, or by ANKRD9-dependent filament-dependent stabilization). On chromatin, IMPDH2 controls nuclear NAD+ availability to modulate PARP1 activity in the DNA damage response. IMPDH2 interacts with multiple partners including cyclophilin A-SFA (via its CBS domain), PARP1 (nuclear), SRPK1, c-Myc, FANCI, B7-H3, and SARS-CoV-2 Nsp14 (mediating NF-κB activation). Disease-associated gain-of-function mutations disrupt GTP-mediated allosteric inhibition by shifting conformational equilibrium toward the active state, causing neurodevelopmental disorders and dystonia, while loss-of-function impairs neural crest and enteric nervous system development."},"narrative":{"mechanistic_narrative":"IMPDH2 catalyzes the rate-limiting, first committed step of de novo guanine nucleotide biosynthesis, oxidizing IMP toward XMP/GTP, and couples this metabolic output to cell proliferation and tissue development [PMID:26546974, PMID:32702086, PMID:38692012]. Its activity is gated by an allosteric mechanism in which GTP binding to regulatory Bateman/CBS domains favors a less active conformation, and reversible polymerization into filaments stabilizes active conformations that resist GTP feedback inhibition, allowing the enzyme to sustain output when guanine nucleotide pools must expand [PMID:31999252]. IMPDH2 is the structural building block of cytoplasmic rod/ring (RR) bodies and co-assembles with CTPS in a manner dictated by which biosynthetic pathway is pharmacologically inhibited [PMID:25601894, PMID:26165495]. Layered post-translational control tunes this output: CDK1 phosphorylates S122 to attenuate catalysis, an inhibition reversed by FGFR-driven PP2A (PPP2CA–PPP2R1A) dephosphorylation that activates the enzyme and promotes S-phase completion [PMID:39739531], while HSPA6-recruited ROCK2 phosphorylates S416 to activate IMPDH2 and limit DNA damage [PMID:42234559]. Enzyme abundance is set by ubiquitin-dependent turnover, with K48-chain removal by USP5 stabilizing the protein, JOSD2 cleaving K63 chains at K134 to restrain IMPDH2-driven NF-κB signaling, and ANKRD9 governing a vesicle-to-filament transition that stabilizes IMPDH2 under nutrient limitation [PMID:40164869, PMID:40177575, PMID:31337707]. Beyond cytoplasmic metabolism, chromatin-associated IMPDH2 binds PARP1 and controls nuclear NAD+ availability to fine-tune the DNA damage response [PMID:39532854]. Gain-of-function point mutations and the S160del variant cause dystonia and neurodevelopmental disorders by shifting the conformational equilibrium toward the active state and disrupting GTP allostery and filament assembly, whereas loss of Impdh2 in neural crest impairs enteric nervous system and craniofacial development [PMID:37414152, PMID:41343675, PMID:26546974].","teleology":[{"year":2009,"claim":"Established that IMPDH2 expression level is set transcriptionally and that a regulatory promoter element controls output, framing the gene as dosage-sensitive before its catalytic regulation was understood.","evidence":"Luciferase promoter-reporter assays of wild-type vs. CRE-disrupting c.-95T>G constructs in HEK293 and Jurkat cells","pmids":["19810816"],"confidence":"Medium","gaps":["Does not identify the trans-acting CRE-binding factor","No link to physiological signaling that engages this promoter element"]},{"year":2010,"claim":"Showed that a coding allozyme reduces enzyme activity chiefly by accelerating protein degradation, introducing protein stability as a determinant of IMPDH2 functional output.","evidence":"Resequencing, allozyme activity and degradation assays, and X-ray crystallography of IMPDH2","pmids":["20718729"],"confidence":"Medium","gaps":["Degradation machinery not identified","Does not connect stability to allosteric regulation"]},{"year":2015,"claim":"Defined IMPDH2 as the essential structural component of cytoplasmic rod/ring bodies and showed assembly composition is dictated by which nucleotide pathway is inhibited, establishing polymerization as a regulated cellular phenomenon.","evidence":"Anti-IMPDH2 antibody microinjection and GFP-IMPDH2 imaging in COS-7; immunofluorescence with ribavirin/MPA/DON and CTPS co-assembly across cell lines","pmids":["25601894","26165495"],"confidence":"Medium","gaps":["Functional consequence of RR assembly on enzyme activity not yet established","Structural basis of filament formation undefined"]},{"year":2015,"claim":"Demonstrated in vivo that IMPDH2-mediated GTP synthesis is required for neural crest development, moving the gene from a housekeeping enzyme to a developmentally essential factor.","evidence":"Wnt1-Cre conditional Impdh2 knockout with lineage tracing and developmental histology in mice","pmids":["26546974"],"confidence":"High","gaps":["Does not resolve which downstream GTP-dependent processes are limiting","Cell-autonomous vs. non-autonomous requirement not dissected"]},{"year":2017,"claim":"Identified IMPDH2 as the isoform-selective intracellular target of the cyclophilin A–sanglifehrin A complex acting through the CBS domain, revealing the regulatory CBS/Bateman region as a ligand-binding hub distinct from the catalytic site.","evidence":"Affinity pulldown, reciprocal Co-IP, CBS-domain mapping, enzymatic assays, and IMPDH1/IMPDH2 isoform selectivity tests","pmids":["28076787"],"confidence":"High","gaps":["Ternary complex does not inhibit catalysis, so mechanism of growth modulation is unresolved","Endogenous physiological ligand of the CBS domain not addressed"]},{"year":2019,"claim":"Connected IMPDH2 abundance to nutrient sensing by showing ANKRD9 drives a vesicle-to-filament transition that switches IMPDH2 between degradation and stabilization, linking polymerization to protein turnover.","evidence":"Reciprocal Co-IP, fractionation, Cys109/110 mutagenesis, ANKRD9 knockdown/overexpression, and guanosine rescue","pmids":["31337707"],"confidence":"High","gaps":["Whether ANKRD9 is the E3 adaptor or a competing stabilizer is not fully resolved","Mechanism coupling filament state to ubiquitin machinery undefined"]},{"year":2020,"claim":"Provided the structural mechanism of filament-based allostery, showing polymerization stabilizes conformations resistant to GTP feedback inhibition—the defining regulatory logic of IMPDH2.","evidence":"Cryo-EM of active and inactive IMPDH2 conformations correlated with enzymatic activity","pmids":["31999252"],"confidence":"High","gaps":["In vivo triggers that initiate polymerization not defined structurally","Does not address post-translational inputs onto the conformational switch"]},{"year":2020,"claim":"Linked IMPDH2-driven GTP supply to anabolic transcription, showing it powers RNA polymerase I/III activity and nucleolar enlargement in cancer cells.","evidence":"IMPDH2 knockdown/overexpression with GTP measurement, Pol I/III activity assays, and nucleolar size analysis in glioblastoma","pmids":["32702086"],"confidence":"Medium","gaps":["Limited methodological detail","Direct vs. indirect coupling of GTP pools to polymerase activity not dissected"]},{"year":2023,"claim":"Established gain-of-function dysregulation as the disease mechanism, showing dystonia/neurodevelopmental mutations shift the conformational equilibrium toward the active state and disrupt GTP allostery.","evidence":"Cryo-EM of a disease mutant plus GTP-inhibition enzymatic assays and patient variant identification","pmids":["37414152"],"confidence":"High","gaps":["Cellular and neuronal consequences of unrestrained activity not directly tested","Genotype–phenotype range across mutations not defined"]},{"year":2023,"claim":"Expanded the IMPDH2 interactome into oncogenic and survival pathways through partners FANCI and B7-H3 that modulate its stability and stress resistance.","evidence":"Co-IP with FANCI/IMPDH2 and B7-H3/IMPDH2, knockdown/rescue, and oxidative stress and proliferation assays in lung cancer cells","pmids":["32021289","37444640"],"confidence":"Medium","gaps":["B7-H3 interaction rests on a single Co-IP without reciprocal validation","Whether these partners act on catalysis or stability is not fully separated"]},{"year":2024,"claim":"Defined a kinase-phosphatase switch on S122, showing CDK1 phosphorylation suppresses activity while FGFR-driven PP2A dephosphorylation activates the enzyme to license S-phase, integrating IMPDH2 into proliferative signaling.","evidence":"In vitro CDK1 kinase assay, phosphoproteomics, S122 mutagenesis, Co-IP of the PPP2CA–PPP2R1A–IMPDH2 complex, and FGFR manipulation","pmids":["39739531"],"confidence":"High","gaps":["Interplay between S122 status and filament assembly not resolved","Whether dephosphorylation acts directly on allostery or catalysis distinct"]},{"year":2024,"claim":"Revealed a nuclear, chromatin-associated role for IMPDH2 in tuning the DNA damage response via PARP1 through control of nuclear NAD+ availability.","evidence":"Chromatin fractionation, IMPDH2–PARP1 Co-IP, nuclear NAD+ quantification, and nuclear-restriction/knockdown DNA-damage assays in TNBC","pmids":["39532854"],"confidence":"High","gaps":["Mechanism by which IMPDH2 reaches chromatin not defined","Whether catalytic activity or scaffolding controls nuclear NAD+ unclear"]},{"year":2024,"claim":"Established that IMPDH2 turnover is set by deubiquitinases, with USP5 removing K48 chains to stabilize the protein and JOSD2 cleaving K63 chains at K134 to restrain its pro-inflammatory NF-κB output.","evidence":"Co-IP, domain mapping, ubiquitin chain-type/site-specific deubiquitination assays, and myeloid JOSD2 knockout colitis model","pmids":["40164869","40177575"],"confidence":"Medium","gaps":["E3 ligases generating these chains not identified","How K63-ubiquitination at K134 enhances NF-κB activity mechanistically unclear"]},{"year":2024,"claim":"Demonstrated tissue-specific developmental and metabolic requirements for IMPDH2 in osteoclast differentiation and adipocyte precursor expansion, the latter mechanistically traced to nucleotide (XMP) provision.","evidence":"LysM-Cre and APC-specific conditional Impdh2 knockouts with transcriptomics, bone-loss and high-fat-diet models, and XMP rescue","pmids":["38959733","38692012"],"confidence":"Medium","gaps":["Whether the osteoclast mitochondrial defect is direct or secondary to nucleotide depletion is unresolved","Cell-type specificity of these requirements not generalized"]},{"year":2024,"claim":"Showed that IMPDH2 filament assembly is physiologically protective and context-dependent, buffering GTP deprivation in neurons and regulating oocyte meiotic competence via MTOR.","evidence":"AMPD2-deficient mouse model with regional filament quantification and dominant-negative IMPDH2 in human neural progenitors; ovarian/cumulus-oocyte imaging with MPA, guanosine rescue, and MTOR inhibition","pmids":["39075237","34124077"],"confidence":"Medium","gaps":["Molecular trigger linking filament state to MTOR signaling not defined","Causal vs. correlative role of filaments in neuroprotection partly inferred"]},{"year":2025,"claim":"Resolved how the S160del neurodevelopmental variant acts, showing it independently disrupts Bateman-domain dimerization (and thus filament assembly) and GTP allostery, separating the two regulatory functions structurally.","evidence":"Cryo-EM of S160del, Xenopus tropicalis expression, purine metabolomics, somitogenesis phenotyping, and high-affinity ligand rescue","pmids":["41343675"],"confidence":"High","gaps":["Mapping of which clinical features arise from filament loss vs. allosteric loss incomplete","Therapeutic correction of allostery not achieved by the rescue ligand"]},{"year":2025,"claim":"Extended IMPDH2 signaling partnerships and viral exploitation, linking it to SRPK1/AKT-mTOR in T cells, c-Myc stability, and SARS-CoV-2 Nsp14-driven NF-κB activation.","evidence":"MS, Co-IP and colocalization for SRPK1; CETSA/Biacore/MST/Co-IP and ubiquitination assays for c-Myc; Nsp14 Co-IP with siRNA/inhibitor NF-κB readouts","pmids":["41213488","39008915","36177032"],"confidence":"Medium","gaps":["Whether these effects require IMPDH2 catalysis or scaffolding is not consistently resolved","Direct vs. indirect nature of the c-Myc and SRPK1 regulatory links unclear"]},{"year":2026,"claim":"Identified an HSPA6–ROCK2 module that activates IMPDH2 via S416 phosphorylation to suppress DNA damage and confer radioresistance, adding a chaperone-kinase activating axis.","evidence":"Co-IP of HSPA6–IMPDH2 and ROCK2 recruitment, in vitro ROCK2 kinase assay on S416, enzymatic activity, and irradiated GBM mouse model","pmids":["42234559"],"confidence":"Medium","gaps":["Interplay of S416 with S122 and Y233 phosphosites not integrated","How HSPA6 selectively recruits ROCK2 to IMPDH2 undefined"]},{"year":null,"claim":"It remains unknown how the multiple regulatory layers—phosphorylation at S122/S416/Y233, lipid (PI3P) binding, ubiquitin turnover, filament assembly, and nuclear relocalization—are integrated into a single coherent control circuit, and which inputs dominate in specific tissues and disease states.","evidence":"","pmids":[],"confidence":"Low","gaps":["No unified model coordinating the distinct phosphosites and allosteric states","Y233 phosphorylation and PI3P regulation rest on a preprint not yet peer-reviewed","Tissue-specific dominance of each regulatory mechanism unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016491","term_label":"oxidoreductase activity","supporting_discovery_ids":[0,3,11,28]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[9,13]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[21]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,9,13]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[5]},{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[5]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,11,28]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[5]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[10]},{"term_id":"R-HSA-162582","term_label":"Signal 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Could also have a single-stranded nucleic acid-binding activity and could play a role in RNA and/or DNA metabolism (PubMed:14766016). It may also have a role in the development of malignancy and the growth progression of some tumors","subcellular_location":"Cytoplasm; Nucleus; Cytoplasm, cytosol","url":"https://www.uniprot.org/uniprotkb/P12268/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/IMPDH2","classification":"Common Essential","n_dependent_lines":620,"n_total_lines":1208,"dependency_fraction":0.5132450331125827},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"PARP1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/IMPDH2","total_profiled":1310},"omim":[{"mim_id":"618605","title":"ANKYRIN REPEAT DOMAIN-CONTAINING PROTEIN 9; ANKRD9","url":"https://www.omim.org/entry/618605"},{"mim_id":"617995","title":"IMPDH2 ENZYME ACTIVITY, VARIATION IN; IMPDH2V","url":"https://www.omim.org/entry/617995"},{"mim_id":"613381","title":"CYSTATHIONINE BETA-SYNTHASE; CBS","url":"https://www.omim.org/entry/613381"},{"mim_id":"603028","title":"TOLL-LIKE RECEPTOR 2; TLR2","url":"https://www.omim.org/entry/603028"},{"mim_id":"164011","title":"NUCLEAR FACTOR KAPPA-B, SUBUNIT 1; NFKB1","url":"https://www.omim.org/entry/164011"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Rods & Rings","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/IMPDH2"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P12268","domains":[{"cath_id":"3.20.20.70","chopping":"19-109_236-494","consensus_level":"high","plddt":94.4012,"start":19,"end":494},{"cath_id":"3.10.580.10","chopping":"126-223","consensus_level":"high","plddt":92.1229,"start":126,"end":223}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P12268","model_url":"https://alphafold.ebi.ac.uk/files/AF-P12268-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P12268-F1-predicted_aligned_error_v6.png","plddt_mean":92.12},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=IMPDH2","jax_strain_url":"https://www.jax.org/strain/search?query=IMPDH2"},"sequence":{"accession":"P12268","fasta_url":"https://rest.uniprot.org/uniprotkb/P12268.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P12268/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P12268"}},"corpus_meta":[{"pmid":"30518405","id":"PMC_30518405","title":"IMPDH2 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Taiwanese patients with isolated or combined dystonia.","date":"2023","source":"Parkinsonism & related disorders","url":"https://pubmed.ncbi.nlm.nih.gov/36657279","citation_count":1,"is_preprint":false},{"pmid":"41445454","id":"PMC_41445454","title":"Glucose Induces DNMT1/IMPDH2-Dependent Metabolic Memory in Endothelial Cells Upon Reprograming Nucleotide Metabolism.","date":"2026","source":"FASEB journal : official publication of the Federation of American Societies for Experimental Biology","url":"https://pubmed.ncbi.nlm.nih.gov/41445454","citation_count":1,"is_preprint":false},{"pmid":"41494804","id":"PMC_41494804","title":"Integrated screens reveal that guanine nucleotide depletion, which is irreversible via targeting IMPDH2, inhibits pancreatic cancer and potentiates KRAS inhibition.","date":"2026","source":"Gut","url":"https://pubmed.ncbi.nlm.nih.gov/41494804","citation_count":1,"is_preprint":false},{"pmid":"40492554","id":"PMC_40492554","title":"Corrigendum to: IMPDH2 Positively 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somitogenesis.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/40654686","citation_count":0,"is_preprint":false},{"pmid":"41343675","id":"PMC_41343675","title":"An IMPDH2 variant associated with neurodevelopmental disorder disrupts purine biosynthesis and somite organization.","date":"2025","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/41343675","citation_count":0,"is_preprint":false},{"pmid":"40350411","id":"PMC_40350411","title":"Domain-specific AI segmentation of IMPDH2 rod/ring structures in mouse embryonic stem cells.","date":"2025","source":"BMC biology","url":"https://pubmed.ncbi.nlm.nih.gov/40350411","citation_count":0,"is_preprint":false},{"pmid":"38328116","id":"PMC_38328116","title":"IMPDH2 filaments protect from neurodegeneration in AMPD2 deficiency.","date":"2024","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/38328116","citation_count":0,"is_preprint":false},{"pmid":"41584327","id":"PMC_41584327","title":"DHL-11, a novel prieurianin-type limonoid isolated from Munronia henryi, targeting IMPDH2 to inhibit triple-negative breast cancer.","date":"2025","source":"Acta pharmaceutica Sinica. 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Filament-dependent allosteric regulation (tuning inhibitor sensitivity) distinguishes IMPDH2 from other metabolic filaments.\",\n      \"method\": \"Cryo-EM structural determination of IMPDH2 in multiple conformational states, correlated with enzymatic activity measurements\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures at multiple conformational states with functional validation of allosteric regulation mechanism in a single rigorous study\",\n      \"pmids\": [\"31999252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IMPDH2 is an intracellular target of the cyclophilin A (PPIA)–sanglifehrin A (SFA) binary complex: the ternary PPIA-SFA-IMPDH2 complex forms through the CBS (cystathionine-β-synthase) domain of IMPDH2, does not inhibit IMPDH2 enzymatic activity, but modulates cell growth. The SFA complex is highly isoform-selective for IMPDH2 over IMPDH1.\",\n      \"method\": \"Affinity pulldown, co-immunoprecipitation, domain mapping (CBS domain), cell growth assays, isoform selectivity experiments\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal interaction identified with pulldown and Co-IP, domain mapped to CBS domain, enzymatic activity tested, isoform selectivity demonstrated in one study with multiple orthogonal methods\",\n      \"pmids\": [\"28076787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"ANKRD9 facilitates degradation of IMPDH2 (the rate-limiting GTP biosynthesis enzyme). Under basal conditions ANKRD9 is segregated from cytosolic IMPDH2 in vesicle-like structures; upon nutrient limitation, ANKRD9 loses its vesicular pattern and assembles with IMPDH2 into rod-like filaments where IMPDH2 is stable. Inhibition of IMPDH2 activity with ribavirin favors ANKRD9 binding to IMPDH2 rods; guanosine supplementation reverses this. The conserved Cys109-Cys110 motif in ANKRD9 is required for the vesicle-to-rods transition and for binding/regulation of IMPDH2. ANKRD9 knockdown increases IMPDH2 levels and prevents IMPDH2 rod formation upon nutrient limitation.\",\n      \"method\": \"Co-immunoprecipitation, fluorescence microscopy, subcellular fractionation, site-directed mutagenesis (Cys109/110), ANKRD9 knockdown and overexpression, guanosine rescue experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, mutagenesis of required motif, multiple rescue conditions, live-cell imaging, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"31337707\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Disease-associated point mutations in IMPDH2 linked to dystonia and neurodevelopmental disorders disrupt GTP-mediated allosteric inhibition of the enzyme. Cryo-EM structures of one IMPDH2 mutant reveal this regulatory defect arises from a shift in the conformational equilibrium toward a more active state, identifying gain-of-function dysregulation as the disease mechanism.\",\n      \"method\": \"Cryo-EM structural determination of mutant IMPDH2, enzymatic activity assays measuring GTP inhibition, patient variant identification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure of disease mutant with enzymatic functional validation; single lab but two orthogonal methods (structure + enzyme assay)\",\n      \"pmids\": [\"37414152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IMPDH2 serine 122 (S122) is phosphorylated by CDK1; this modification attenuates IMPDH2 catalytic activity for IMP oxidation and represses its allosteric modulation by purine nucleotides. FGFR signaling activation triggers IMPDH2-S122 dephosphorylation mediated by protein phosphatase 2A (PP2A); this is dependent on FGFR3-mediated phosphorylation of PPP2R1A at Y261, leading to formation of a PPP2CA–PPP2R1A–IMPDH2 complex. Dephosphorylation at S122 activates IMPDH2, promotes guanine nucleotide synthesis, facilitates S-phase completion and cell proliferation.\",\n      \"method\": \"In vitro kinase assay (CDK1), phosphoproteomics, co-immunoprecipitation (PP2A complex), site-directed mutagenesis (S122), enzymatic activity assays, FGFR signaling manipulation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro kinase assay, mutagenesis, co-IP of phosphatase complex, enzymatic activity assays; multiple orthogonal methods in single study\",\n      \"pmids\": [\"39739531\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IMPDH2 is enriched on chromatin in triple-negative breast cancer (TNBC). On chromatin, IMPDH2 interacts with PARP1 and modulates PARP1 activity by controlling the nuclear availability of NAD+, fine-tuning the DNA damage response. IMPDH2 repression leads to DNA damage accumulation; when IMPDH2 is restricted to the nucleus, it depletes nuclear NAD+, leading to PARP1 cleavage and cell death.\",\n      \"method\": \"Chromatin fractionation, co-immunoprecipitation (IMPDH2-PARP1), NAD+ measurement in nuclear fractions, IMPDH2 knockdown/nuclear restriction experiments, DNA damage assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP, subcellular fractionation, NAD+ quantification, KD with specific phenotypic readout; multiple orthogonal methods in single study\",\n      \"pmids\": [\"39532854\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"USP5 (ubiquitin-specific protease 5) binds IMPDH2 through its N-terminal cryptic ZnF-UBP and ZnF-UBP domains interacting with IMPDH2 residues 251–514, and removes K48-linked ubiquitin chains from IMPDH2, preventing ubiquitin-mediated proteasomal degradation and stabilizing IMPDH2 protein levels.\",\n      \"method\": \"Co-immunoprecipitation, domain mapping, ubiquitination assay, USP5 knockdown/overexpression, proteasome inhibitor experiments\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP with domain mapping, ubiquitination assay showing K48 chain removal, single lab\",\n      \"pmids\": [\"40164869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"JOSD2 (a deubiquitinating enzyme) binds the C-terminal domain of IMPDH2 and preferentially cleaves K63-linked polyubiquitin chains at the K134 site of IMPDH2, suppressing IMPDH2 activity and preventing activation of NF-κB signaling and inflammation in macrophages. JOSD2 deficiency exacerbates colitis through enhanced IMPDH2-mediated NF-κB activation.\",\n      \"method\": \"Co-immunoprecipitation, in vitro deubiquitination assay (K63 chain specificity, K134 site identification), myeloid-specific JOSD2 knockout mice, NF-κB reporter assays\",\n      \"journal\": \"Acta pharmaceutica Sinica. B\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, in vitro deubiquitination assay with ubiquitin chain type and site specificity identified, conditional KO mouse model; single lab\",\n      \"pmids\": [\"40177575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SARS-CoV-2 Nsp14 protein physically interacts with IMPDH2, and IMPDH2 knockdown or chemical inhibition abolishes Nsp14-mediated NF-κB activation and downstream cytokine (IL-6, IL-8) induction, demonstrating that IMPDH2 is required for Nsp14-driven NF-κB signaling and is exploited for viral replication.\",\n      \"method\": \"Co-immunoprecipitation (Nsp14–IMPDH2 interaction), siRNA knockdown of IMPDH2, NF-κB reporter assays, IMPDH2 inhibitors (ribavirin, mycophenolic acid), viral replication assays\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP confirmed interaction, genetic and pharmacological inhibition with specific signaling readout; single lab, multiple methods\",\n      \"pmids\": [\"36177032\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Microinjection of affinity-purified anti-IMPDH2 antibodies into live COS-7 cells caused disassembly of rod/ring (RR) structures, demonstrating that IMPDH2 is a major structural component and essential building block of cytoplasmic RR structures. RR structures are primarily stationary and stable in live cells. GFP-tagged IMPDH2 confirmed these findings independently.\",\n      \"method\": \"Microinjection of anti-IMPDH2 antibodies in live cells, live-cell fluorescence microscopy, GFP-IMPDH2 transfection, time-lapse imaging\",\n      \"journal\": \"Cell & bioscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional manipulation (antibody microinjection) in live cells with real-time imaging, two independent methods (antibody injection and GFP fusion); single lab\",\n      \"pmids\": [\"25601894\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Conditional deletion of Impdh2 in early neural crest cells (using Wnt1-Cre) produces highly penetrant intestinal aganglionosis, agenesis of craniofacial skeleton, and cardiac outflow tract/great vessel malformations, establishing that IMPDH2-mediated guanine nucleotide synthesis is essential for enteric nervous system development and other neural crest derivatives.\",\n      \"method\": \"Conditional knockout mouse (Wnt1-Cre × Impdh2-flox), Rosa26 reporter lineage tracing, histological and developmental analysis\",\n      \"journal\": \"Developmental biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — tissue-specific conditional KO with defined neural crest phenotypes, lineage tracing; well-controlled in vivo genetics\",\n      \"pmids\": [\"26546974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"IMPDH2 expression is necessary for activation of de novo GTP biosynthesis in glioblastoma; increased IMPDH2 enhances RNA polymerase I and III transcription, directly linking GTP metabolism to anabolic capacity and nucleolar enlargement.\",\n      \"method\": \"IMPDH2 knockdown/overexpression, GTP level measurement, RNA polymerase I/III activity assays, nucleolar size analysis\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — KD with specific metabolic and transcriptional phenotype, single lab review-style paper summarizing own findings; limited methodological detail in abstract\",\n      \"pmids\": [\"32702086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FANCI directly binds IMPDH2 (by co-immunoprecipitation) and decreases IMPDH2 degradation; overexpression of IMPDH2 reverses the inhibitory effects of FANCI knockdown on lung adenocarcinoma cell proliferation via MEK/ERK/MMPs signaling activation.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence co-localization, FANCI knockdown, IMPDH2 overexpression rescue, western blotting of MEK/ERK/MMP pathway\",\n      \"journal\": \"OncoTargets and therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single Co-IP with functional rescue experiment; pathway placement by western blot; single lab\",\n      \"pmids\": [\"32021289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"IMPDH2 and CTPS co-assemble into cytoplasmic rod/ring (RR) structures in a cell-type- and induction-dependent manner. Ribavirin and mycophenolic acid (MPA) treatments induce exclusively IMPDH2-based RR, while DON treatment produces mixed IMPDH2/CTPS RR. Assembly composition is determined by which biosynthetic pathway is inhibited.\",\n      \"method\": \"Immunofluorescence microscopy, drug treatments (ribavirin, MPA, DON), HA-tagged CTPS1 transfection, quantification of RR composition across cell lines\",\n      \"journal\": \"Journal of genetics and genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct imaging with multiple pharmacological conditions and transfection approaches; replicated across two cell types (HeLa and COS-7)\",\n      \"pmids\": [\"26165495\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In mouse models of AMPD2 deficiency, neurodegeneration-resistant brain regions accumulate micron-sized IMPDH2 filaments while the vulnerable hippocampal dentate gyrus shows barely detectable filaments. Blocking IMPDH2 polymerization using a dominant-negative IMPDH2 variant impairs growth of AMPD2-deficient neural progenitor cells, suggesting IMPDH2 filament assembly protects against GTP deprivation-induced neurodegeneration.\",\n      \"method\": \"Mouse knockout model (AMPD2 deficiency), immunofluorescence for IMPDH2 filaments, dominant-negative IMPDH2 variant expression in human neural progenitor cells, GTP level measurement\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo mouse model with regional filament quantification and human cell dominant-negative experiment; single lab\",\n      \"pmids\": [\"39075237\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"EBV EBNA2 and MYC (but not LMP1) induce IMPDH2 gene expression during primary B cell infection, leading to nucleolar hypertrophy. IMPDH2 induction and nucleolar enlargement are required for EBV-driven growth transformation of primary B cells, as inhibition by mycophenolic acid (MPA) blocks transformation.\",\n      \"method\": \"RNA-seq (IMPDH2 induction kinetics), EBNA2/LMP1 knockout EBV viruses, MPA pharmacological inhibition, electron microscopy of nucleolar size, mouse xenograft model with oral MMF\",\n      \"journal\": \"Microbiology spectrum\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic (viral KO) and pharmacological approaches to define upstream regulator and functional requirement; multiple methods in single lab\",\n      \"pmids\": [\"37409959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"IMPDH2 interacts with SRPK1 in CD4+ T cells (confirmed by mass spectrometry, co-immunoprecipitation, and confocal colocalization); IMPDH2 regulates SRPK1 expression, which subsequently activates the AKT/mTOR signaling pathway to promote CD4+ T cell proliferation and suppress apoptosis in myasthenia gravis.\",\n      \"method\": \"Mass spectrometry, co-immunoprecipitation, confocal fluorescence microscopy, IMPDH2 knockdown/overexpression, western blotting of AKT/mTOR pathway, flow cytometry\",\n      \"journal\": \"Clinical immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — MS + Co-IP + colocalization identify SRPK1 as interactor; pathway placement by western blot; single lab\",\n      \"pmids\": [\"41213488\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Shikonin is a competitive inhibitor of IMPDH2 (Lineweaver-Burk plot), directly interacts with IMPDH2 protein (thermal shift assay and fluorescence quenching), and its anti-proliferative effects in TNBC cells are rescued by exogenous guanosine supplementation, confirming on-target mechanism through GTP biosynthesis inhibition.\",\n      \"method\": \"Enzymatic inhibition assays (Lineweaver-Burk), thermal shift assay, fluorescence quenching, molecular docking, guanosine rescue in cell proliferation assays, IMPDH2 knockdown\",\n      \"journal\": \"Phytotherapy research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro enzymatic assay with mechanism determination, biophysical binding assays, guanosine rescue; multiple orthogonal methods; single lab\",\n      \"pmids\": [\"32779300\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The IMPDH2 allozyme Phe263 has decreased enzymatic activity primarily due to decreased protein quantity caused by accelerated protein degradation, as supported by X-ray crystal structure analysis of IMPDH2. A promoter SNP (G>C IVS1-93) is associated with decreased mRNA quantity, possibly through altered transcription.\",\n      \"method\": \"Resequencing, allozyme functional assays, protein degradation experiments, X-ray crystal structure of IMPDH2, real-time PCR for mRNA quantification\",\n      \"journal\": \"British journal of pharmacology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure available plus protein stability assays; functional consequence of variant defined mechanistically; single lab\",\n      \"pmids\": [\"20718729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A promoter SNP in IMPDH2 (c.-95T>G) disrupts a cAMP response element (CRE) binding site, leading to a 55–65% decrease in IMPDH2 transcriptional activity as measured by luciferase reporter assays in HEK293 and Jurkat cells.\",\n      \"method\": \"Luciferase reporter gene assays with IMPDH2 promoter constructs (wild-type vs. mutant) transfected into HEK293 and Jurkat cell lines\",\n      \"journal\": \"Genetic testing and molecular biomarkers\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct promoter-reporter functional assay in two cell lines; single lab, single method type\",\n      \"pmids\": [\"19810816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IMPDH2 cytoophidia (filaments) form in mouse oocytes in vivo during growth and in vitro upon IMPDH inhibition with MPA. In granulosa cells, cytoophidia form after the LH surge coinciding with reduced IMPDH2 protein expression and resumption of oocyte meiosis. MPA-induced cytoophidium formation in cumulus cells triggers meiosis resumption in enclosed oocytes via the MTOR pathway; guanosine supplementation prevents this, linking IMPDH2 filament formation to regulation of oocyte meiotic competence.\",\n      \"method\": \"Immunofluorescence in mouse ovarian sections, MPA treatment of cumulus-oocyte complexes (COCs), LH surge induction, guanosine rescue, MTOR pathway inhibition, meiotic resumption assay\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo and in vitro imaging with pharmacological manipulation and pathway rescue; single lab with multiple conditions\",\n      \"pmids\": [\"34124077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ALK and SRC kinases phosphorylate IMPDH2 at tyrosine 233 (Y233) within the Bateman domain, and this phosphorylation is essential for IMPDH2 enzymatic activity. Phosphoinositide-3-phosphate (PI3P) selectively binds to and inhibits IMPDH2 but not IMPDH1, revealing an isoform-specific lipid-mediated regulatory mechanism.\",\n      \"method\": \"Peptide-based mutagenesis, in vitro enzymatic assays, PI3P binding assay, kinase activity assays (ALK, SRC), isoform comparison (IMPDH1 vs IMPDH2)\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 1 / Weak — preprint with in vitro enzymatic and binding assays; not yet peer-reviewed; single study\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Impdh2 deletion in mouse myeloid lineage cells (LysM-Cre conditional KO) reduces osteoclast number and impairs osteoclast differentiation marker gene expression (Nfatc1, Ctsk, Calcr, Acp5, Dcstamp, Atp6v0d2), with genome-wide analysis showing impairment of mitochondrial biogenesis and oxidative phosphorylation in Impdh2-deficient osteoclasts. Deletion alleviates ovariectomy-induced bone loss.\",\n      \"method\": \"Myeloid-specific conditional KO (Impdh2-LysM-Cre), histomorphometric analysis, qPCR/western blot, genome-wide transcriptomic analysis, ovariectomy mouse model\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-type-specific conditional KO with defined osteoclast phenotype, genome-wide pathway analysis, in vivo bone loss model; single lab\",\n      \"pmids\": [\"38959733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Vitexicarpin directly binds IMPDH2 (confirmed by CETSA, Biacore, MST) and promotes c-Myc ubiquitination by disrupting the protein-protein interaction between IMPDH2 and c-Myc, thereby inhibiting EMT in colorectal cancer. Co-IP confirmed a direct IMPDH2–c-Myc interaction.\",\n      \"method\": \"CETSA, label-free quantitative proteomics, Biacore, Co-IP, MST, ubiquitination assay, molecular docking, IMPDH2 overexpression/knockdown\",\n      \"journal\": \"Phytomedicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biophysical binding methods plus Co-IP for IMPDH2-c-Myc interaction; functional consequence via ubiquitination assay; single lab\",\n      \"pmids\": [\"39008915\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"HSPA6 interacts with IMPDH2 and activates it by recruiting ROCK2 to phosphorylate IMPDH2 at serine 416 (S416), thereby promoting GTP synthesis, reducing DNA damage, and enhancing radioresistance of glioblastoma stem cells.\",\n      \"method\": \"Co-immunoprecipitation (HSPA6–IMPDH2, ROCK2 recruitment), in vitro kinase assay (ROCK2 phosphorylation of IMPDH2 S416), enzymatic activity assays, IMPDH2 knockdown/inhibition with irradiation, mouse GBM model\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — kinase assay with identified phosphorylation site, Co-IP of complex, enzymatic activity, in vivo validation; single lab with multiple methods\",\n      \"pmids\": [\"42234559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"IMPDH2 S160del mutant (neurodevelopmental disorder-associated) disrupts in vivo IMPDH2 filament assembly in Xenopus tropicalis by destabilizing dimerization of regulatory Bateman domains, as shown by cryo-EM structures. S160del expression disrupts purine pools and somite organization. Restoring Bateman domain dimerization with a high-affinity ligand rescues filament formation but does not restore GTP inhibition sensitivity, indicating S160del disrupts both filament assembly and allosteric regulation independently.\",\n      \"method\": \"Cryo-EM structure of S160del mutant, Xenopus tropicalis expression (WT vs. S160del), metabolomics (purine pools), developmental phenotyping (somitogenesis), filament visualization, high-affinity ligand rescue\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure, in vivo vertebrate model, metabolomics, ligand rescue; multiple orthogonal methods; single lab with rigorous design\",\n      \"pmids\": [\"41343675\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IMPDH2 forms localized spots (not diffuse filaments) predominantly at branching sites and distal ends of astrocyte stem processes in adult mouse hippocampus and primary astrocyte cultures, suggesting a role in site-specific GTP availability regulation at specialized astrocyte nodes.\",\n      \"method\": \"Immunofluorescence microscopy of adult mouse hippocampal sections and primary astrocyte cultures, quantitative distribution analysis\",\n      \"journal\": \"Genes to cells\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — localization by immunofluorescence only; functional consequence not directly tested; single lab, single method\",\n      \"pmids\": [\"38009721\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In Xenopus tropicalis tadpoles, inhibition of IMPDH2 impairs tail regeneration and reduces cell proliferation in regenerating tissue. Endogenous and GFP-tagged IMPDH2 robustly assembles into filaments throughout the tadpole tail, and the regenerating tail creates a sensitized condition for filament formation compared to non-regenerating tissue.\",\n      \"method\": \"IMPDH2 inhibitor treatment during tail regeneration, proliferation assay (EdU), endogenous filament immunofluorescence, GFP-IMPDH2 fusion live imaging in tadpole tail\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 2 / Weak — preprint; direct inhibition with proliferation readout and filament imaging in vivo; not yet peer-reviewed\",\n      \"pmids\": [\"39131357\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Impdh2 conditional knockout in adipocyte precursor cells (APCs) reduces white adipose tissue expansion under high-fat diet by limiting mitotic clonal expansion (MCE) during adipogenesis. XMP (xanthosine monophosphate, an IMPDH2 product) enhances adipogenic potential; exogenous XMP rescues the reduced adipogenic ability of Impdh2-deficient APCs, mechanistically linking Impdh2 activity to APC proliferation through nucleotide provision.\",\n      \"method\": \"APC-specific conditional Impdh2 knockout, high-fat diet model, XMP supplementation rescue, adipogenesis assays, energy expenditure measurement\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO with metabolite rescue experiment defining mechanism; single lab, in vivo and in vitro\",\n      \"pmids\": [\"38692012\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"B7-H3 (CD276) co-immunoprecipitates with IMPDH2 in lung cancer cells; this complex protects cancer cells from oxidative stress triggered by chemotherapy, promoting cell survival. Loss of B7-H3 promotes 3D spheroid expansion in an IMPDH2-dependent manner, establishing a functional interaction between B7-H3 and IMPDH2 in metabolic homeostasis.\",\n      \"method\": \"Co-immunoprecipitation (B7-H3–IMPDH2), oxidative stress assays, B7-H3 knockdown with 3D spheroid growth, IMPDH2-dependent rescue experiments\",\n      \"journal\": \"Cancers\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP with limited mechanistic follow-up; single lab, single interaction method\",\n      \"pmids\": [\"37444640\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IMPDH2 is the rate-limiting enzyme catalyzing the first committed step in de novo guanine nucleotide (GTP) biosynthesis; it is allosterically inhibited by GTP and regulated by reversible polymerization into filaments that reduce sensitivity to GTP feedback inhibition. Its activity is controlled by multiple post-translational mechanisms: CDK1 phosphorylates S122 to attenuate activity (reversed by FGFR-PP2A signaling), ROCK2 (recruited by HSPA6) phosphorylates S416 to activate it, ALK/SRC phosphorylate Y233 for activity, and K48-linked ubiquitination targets it for proteasomal degradation (counteracted by deubiquitinases USP5 and JOSD2, or by ANKRD9-dependent filament-dependent stabilization). On chromatin, IMPDH2 controls nuclear NAD+ availability to modulate PARP1 activity in the DNA damage response. IMPDH2 interacts with multiple partners including cyclophilin A-SFA (via its CBS domain), PARP1 (nuclear), SRPK1, c-Myc, FANCI, B7-H3, and SARS-CoV-2 Nsp14 (mediating NF-κB activation). Disease-associated gain-of-function mutations disrupt GTP-mediated allosteric inhibition by shifting conformational equilibrium toward the active state, causing neurodevelopmental disorders and dystonia, while loss-of-function impairs neural crest and enteric nervous system development.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"IMPDH2 catalyzes the rate-limiting, first committed step of de novo guanine nucleotide biosynthesis, oxidizing IMP toward XMP/GTP, and couples this metabolic output to cell proliferation and tissue development [#10, #11, #28]. Its activity is gated by an allosteric mechanism in which GTP binding to regulatory Bateman/CBS domains favors a less active conformation, and reversible polymerization into filaments stabilizes active conformations that resist GTP feedback inhibition, allowing the enzyme to sustain output when guanine nucleotide pools must expand [#0]. IMPDH2 is the structural building block of cytoplasmic rod/ring (RR) bodies and co-assembles with CTPS in a manner dictated by which biosynthetic pathway is pharmacologically inhibited [#9, #13]. Layered post-translational control tunes this output: CDK1 phosphorylates S122 to attenuate catalysis, an inhibition reversed by FGFR-driven PP2A (PPP2CA–PPP2R1A) dephosphorylation that activates the enzyme and promotes S-phase completion [#4], while HSPA6-recruited ROCK2 phosphorylates S416 to activate IMPDH2 and limit DNA damage [#24]. Enzyme abundance is set by ubiquitin-dependent turnover, with K48-chain removal by USP5 stabilizing the protein, JOSD2 cleaving K63 chains at K134 to restrain IMPDH2-driven NF-\\u03baB signaling, and ANKRD9 governing a vesicle-to-filament transition that stabilizes IMPDH2 under nutrient limitation [#6, #7, #2]. Beyond cytoplasmic metabolism, chromatin-associated IMPDH2 binds PARP1 and controls nuclear NAD+ availability to fine-tune the DNA damage response [#5]. Gain-of-function point mutations and the S160del variant cause dystonia and neurodevelopmental disorders by shifting the conformational equilibrium toward the active state and disrupting GTP allostery and filament assembly, whereas loss of Impdh2 in neural crest impairs enteric nervous system and craniofacial development [#3, #25, #10].\",\n  \"teleology\": [\n    {\n      \"year\": 2009,\n      \"claim\": \"Established that IMPDH2 expression level is set transcriptionally and that a regulatory promoter element controls output, framing the gene as dosage-sensitive before its catalytic regulation was understood.\",\n      \"evidence\": \"Luciferase promoter-reporter assays of wild-type vs. CRE-disrupting c.-95T>G constructs in HEK293 and Jurkat cells\",\n      \"pmids\": [\"19810816\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not identify the trans-acting CRE-binding factor\", \"No link to physiological signaling that engages this promoter element\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Showed that a coding allozyme reduces enzyme activity chiefly by accelerating protein degradation, introducing protein stability as a determinant of IMPDH2 functional output.\",\n      \"evidence\": \"Resequencing, allozyme activity and degradation assays, and X-ray crystallography of IMPDH2\",\n      \"pmids\": [\"20718729\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Degradation machinery not identified\", \"Does not connect stability to allosteric regulation\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defined IMPDH2 as the essential structural component of cytoplasmic rod/ring bodies and showed assembly composition is dictated by which nucleotide pathway is inhibited, establishing polymerization as a regulated cellular phenomenon.\",\n      \"evidence\": \"Anti-IMPDH2 antibody microinjection and GFP-IMPDH2 imaging in COS-7; immunofluorescence with ribavirin/MPA/DON and CTPS co-assembly across cell lines\",\n      \"pmids\": [\"25601894\", \"26165495\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of RR assembly on enzyme activity not yet established\", \"Structural basis of filament formation undefined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated in vivo that IMPDH2-mediated GTP synthesis is required for neural crest development, moving the gene from a housekeeping enzyme to a developmentally essential factor.\",\n      \"evidence\": \"Wnt1-Cre conditional Impdh2 knockout with lineage tracing and developmental histology in mice\",\n      \"pmids\": [\"26546974\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not resolve which downstream GTP-dependent processes are limiting\", \"Cell-autonomous vs. non-autonomous requirement not dissected\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identified IMPDH2 as the isoform-selective intracellular target of the cyclophilin A\\u2013sanglifehrin A complex acting through the CBS domain, revealing the regulatory CBS/Bateman region as a ligand-binding hub distinct from the catalytic site.\",\n      \"evidence\": \"Affinity pulldown, reciprocal Co-IP, CBS-domain mapping, enzymatic assays, and IMPDH1/IMPDH2 isoform selectivity tests\",\n      \"pmids\": [\"28076787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ternary complex does not inhibit catalysis, so mechanism of growth modulation is unresolved\", \"Endogenous physiological ligand of the CBS domain not addressed\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Connected IMPDH2 abundance to nutrient sensing by showing ANKRD9 drives a vesicle-to-filament transition that switches IMPDH2 between degradation and stabilization, linking polymerization to protein turnover.\",\n      \"evidence\": \"Reciprocal Co-IP, fractionation, Cys109/110 mutagenesis, ANKRD9 knockdown/overexpression, and guanosine rescue\",\n      \"pmids\": [\"31337707\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether ANKRD9 is the E3 adaptor or a competing stabilizer is not fully resolved\", \"Mechanism coupling filament state to ubiquitin machinery undefined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Provided the structural mechanism of filament-based allostery, showing polymerization stabilizes conformations resistant to GTP feedback inhibition\\u2014the defining regulatory logic of IMPDH2.\",\n      \"evidence\": \"Cryo-EM of active and inactive IMPDH2 conformations correlated with enzymatic activity\",\n      \"pmids\": [\"31999252\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo triggers that initiate polymerization not defined structurally\", \"Does not address post-translational inputs onto the conformational switch\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linked IMPDH2-driven GTP supply to anabolic transcription, showing it powers RNA polymerase I/III activity and nucleolar enlargement in cancer cells.\",\n      \"evidence\": \"IMPDH2 knockdown/overexpression with GTP measurement, Pol I/III activity assays, and nucleolar size analysis in glioblastoma\",\n      \"pmids\": [\"32702086\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Limited methodological detail\", \"Direct vs. indirect coupling of GTP pools to polymerase activity not dissected\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Established gain-of-function dysregulation as the disease mechanism, showing dystonia/neurodevelopmental mutations shift the conformational equilibrium toward the active state and disrupt GTP allostery.\",\n      \"evidence\": \"Cryo-EM of a disease mutant plus GTP-inhibition enzymatic assays and patient variant identification\",\n      \"pmids\": [\"37414152\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular and neuronal consequences of unrestrained activity not directly tested\", \"Genotype\\u2013phenotype range across mutations not defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Expanded the IMPDH2 interactome into oncogenic and survival pathways through partners FANCI and B7-H3 that modulate its stability and stress resistance.\",\n      \"evidence\": \"Co-IP with FANCI/IMPDH2 and B7-H3/IMPDH2, knockdown/rescue, and oxidative stress and proliferation assays in lung cancer cells\",\n      \"pmids\": [\"32021289\", \"37444640\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"B7-H3 interaction rests on a single Co-IP without reciprocal validation\", \"Whether these partners act on catalysis or stability is not fully separated\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Defined a kinase-phosphatase switch on S122, showing CDK1 phosphorylation suppresses activity while FGFR-driven PP2A dephosphorylation activates the enzyme to license S-phase, integrating IMPDH2 into proliferative signaling.\",\n      \"evidence\": \"In vitro CDK1 kinase assay, phosphoproteomics, S122 mutagenesis, Co-IP of the PPP2CA\\u2013PPP2R1A\\u2013IMPDH2 complex, and FGFR manipulation\",\n      \"pmids\": [\"39739531\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Interplay between S122 status and filament assembly not resolved\", \"Whether dephosphorylation acts directly on allostery or catalysis distinct\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Revealed a nuclear, chromatin-associated role for IMPDH2 in tuning the DNA damage response via PARP1 through control of nuclear NAD+ availability.\",\n      \"evidence\": \"Chromatin fractionation, IMPDH2\\u2013PARP1 Co-IP, nuclear NAD+ quantification, and nuclear-restriction/knockdown DNA-damage assays in TNBC\",\n      \"pmids\": [\"39532854\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which IMPDH2 reaches chromatin not defined\", \"Whether catalytic activity or scaffolding controls nuclear NAD+ unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Established that IMPDH2 turnover is set by deubiquitinases, with USP5 removing K48 chains to stabilize the protein and JOSD2 cleaving K63 chains at K134 to restrain its pro-inflammatory NF-\\u03baB output.\",\n      \"evidence\": \"Co-IP, domain mapping, ubiquitin chain-type/site-specific deubiquitination assays, and myeloid JOSD2 knockout colitis model\",\n      \"pmids\": [\"40164869\", \"40177575\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"E3 ligases generating these chains not identified\", \"How K63-ubiquitination at K134 enhances NF-\\u03baB activity mechanistically unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrated tissue-specific developmental and metabolic requirements for IMPDH2 in osteoclast differentiation and adipocyte precursor expansion, the latter mechanistically traced to nucleotide (XMP) provision.\",\n      \"evidence\": \"LysM-Cre and APC-specific conditional Impdh2 knockouts with transcriptomics, bone-loss and high-fat-diet models, and XMP rescue\",\n      \"pmids\": [\"38959733\", \"38692012\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the osteoclast mitochondrial defect is direct or secondary to nucleotide depletion is unresolved\", \"Cell-type specificity of these requirements not generalized\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Showed that IMPDH2 filament assembly is physiologically protective and context-dependent, buffering GTP deprivation in neurons and regulating oocyte meiotic competence via MTOR.\",\n      \"evidence\": \"AMPD2-deficient mouse model with regional filament quantification and dominant-negative IMPDH2 in human neural progenitors; ovarian/cumulus-oocyte imaging with MPA, guanosine rescue, and MTOR inhibition\",\n      \"pmids\": [\"39075237\", \"34124077\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular trigger linking filament state to MTOR signaling not defined\", \"Causal vs. correlative role of filaments in neuroprotection partly inferred\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Resolved how the S160del neurodevelopmental variant acts, showing it independently disrupts Bateman-domain dimerization (and thus filament assembly) and GTP allostery, separating the two regulatory functions structurally.\",\n      \"evidence\": \"Cryo-EM of S160del, Xenopus tropicalis expression, purine metabolomics, somitogenesis phenotyping, and high-affinity ligand rescue\",\n      \"pmids\": [\"41343675\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mapping of which clinical features arise from filament loss vs. allosteric loss incomplete\", \"Therapeutic correction of allostery not achieved by the rescue ligand\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extended IMPDH2 signaling partnerships and viral exploitation, linking it to SRPK1/AKT-mTOR in T cells, c-Myc stability, and SARS-CoV-2 Nsp14-driven NF-\\u03baB activation.\",\n      \"evidence\": \"MS, Co-IP and colocalization for SRPK1; CETSA/Biacore/MST/Co-IP and ubiquitination assays for c-Myc; Nsp14 Co-IP with siRNA/inhibitor NF-\\u03baB readouts\",\n      \"pmids\": [\"41213488\", \"39008915\", \"36177032\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether these effects require IMPDH2 catalysis or scaffolding is not consistently resolved\", \"Direct vs. indirect nature of the c-Myc and SRPK1 regulatory links unclear\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Identified an HSPA6\\u2013ROCK2 module that activates IMPDH2 via S416 phosphorylation to suppress DNA damage and confer radioresistance, adding a chaperone-kinase activating axis.\",\n      \"evidence\": \"Co-IP of HSPA6\\u2013IMPDH2 and ROCK2 recruitment, in vitro ROCK2 kinase assay on S416, enzymatic activity, and irradiated GBM mouse model\",\n      \"pmids\": [\"42234559\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Interplay of S416 with S122 and Y233 phosphosites not integrated\", \"How HSPA6 selectively recruits ROCK2 to IMPDH2 undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unknown how the multiple regulatory layers\\u2014phosphorylation at S122/S416/Y233, lipid (PI3P) binding, ubiquitin turnover, filament assembly, and nuclear relocalization\\u2014are integrated into a single coherent control circuit, and which inputs dominate in specific tissues and disease states.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No unified model coordinating the distinct phosphosites and allosteric states\", \"Y233 phosphorylation and PI3P regulation rest on a preprint not yet peer-reviewed\", \"Tissue-specific dominance of each regulatory mechanism unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016491\", \"supporting_discovery_ids\": [0, 3, 11, 28]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [9, 13]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [21]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 9, 13]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 11, 28]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4, 16, 24]}\n    ],\n    \"complexes\": [\n      \"rod/ring (RR) cytoophidia filaments\",\n      \"PPP2CA\\u2013PPP2R1A\\u2013IMPDH2 (PP2A) complex\",\n      \"PPIA\\u2013SFA\\u2013IMPDH2 ternary complex\"\n    ],\n    \"partners\": [\n      \"PARP1\",\n      \"ANKRD9\",\n      \"USP5\",\n      \"JOSD2\",\n      \"PPIA\",\n      \"SRPK1\",\n      \"FANCI\",\n      \"HSPA6\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}